In the past microwave devices such as Monolithic Microwave Integrated Circuits (MMICs) have been utilized in a wide variety of RF power amplifier applications and more specifically in the military for towed or expendable jammers that use a large number of wideband high power solid-state amplifying devices. Such amplifiers and their applications are discussed in U.S. Pat. Nos. 7,924,097 and 8,076,975 issued to Robert Actis et al., assigned to the assigned hereof and incorporated by reference. In these devices multiple transistors are placed on a substrate and in general each generate 2 watts of RF power with an associated 2 watts of DC power that must be dissipated. The problem when large numbers of high power transistors are used in amplifiers is the ability of the substrate to dissipate the heat that is generated, generally at the gate electrodes of the high power transistors.
It has been found that the silicon carbide host substrate of GaN transistors with its high thermal conductivity provides an excellent way of transferring the heat away from the vicinity of the gate electrode to a heat sink, whereby the approximate 2 watts of waste heat that is generated in the operation of the power transistor is effectively dissipated.
High power gallium nitride transistors commonly have their gallium nitride layer on top of a silicon carbide substrate, the bottom of which is metalized and in contact with a heat sink that provides an excellent thermal ground plane structure for an array of high power MMIC RF amplifiers.
The problem with such amplifiers is the fact that the RF energy is applied to or coupled out of the transistor utilizing wire bonds. However, wire bonding techniques are non-optimal due to discontinuities at the points of the attachment of the wire bond where power is lost and reliability is sacrificed. Aside from the awkwardness of having to provide a number of wire bonds, the associated parasitics of multiple wire bonds limits the usable bandwidth performance of the wideband amplifying MMICs.
Thus, while silicon carbide as a substrate has excellent thermal conductivity to be able to dissipate the heat in the vicinity of the gate structure for high power transistors, the use of wire bonds to contact the gallium nitride transistors make the use of these wire bonded high power amplifiers less desirable from an optimal power-bandwidth performance perspective.
It will be noted that in these types of devices the power level for each individual 400 micron device with a power density of 5 watts per millimeter results in the generation of 2 to 3 watts of RF power. When 40-90 of these individual devices are placed in a circuit, the amount of heat that must be dissipated is for instance 90 times that associated with a single one of these devices. While the heat associated with the above devices is associated with RF power, one nonetheless has to dissipate DC currents, and for a 50% efficient device, one needs for instance to dissipate 2 watts of waste heat for every 2 watts of RF output power.
In order to be able to use the GaN on silicon carbide semiconductor technology, it has been suggested to utilize a flip chip die attach approach which is a very cost effective technique for connecting to integrated circuits.
However, when amplifiers in the form of MMICs are to be connected utilizing solder balls and flip chip attachment techniques, there is a problem because when the chip is flipped upside down such that the silicon carbide base now has a top surface open to the air, all of the heat dissipated through the silicon carbide has nowhere to go as there is no heat sink available at this top surface of the silicon carbide. It is noted that power flip chip components may have additional solder pads or metal bumps in the vicinity of the heat generating components, e.g. the source contacts of a field effect transistor to dissipate heat. However, this complicates the design of the module base, limits the circuit architecture that can be applied to the modular integrated circuit and may compromise then al management.
In summary, while flip chip attachment of RF semiconductor components is often desired for the purpose of eliminating wire bonds and the accompanying ill defined parasitics, it is difficult to draw heat out of the chip in an efficient manner due to the lack of a heat sink on top of the bare silicon carbide substrate.